The Girl Next Door is probably the most disturbing book that I have ever read. The horrors depicted in the book almost all come directly from the Sylvia Likens murder case which took place in Indiana in the 1960's.
Given the extreme nature of the case that the book is based upon, it sometimes reads as sensational, or even exploitational, but the facts of the case add an element to the story that makes it incredibly difficult to dismiss.
Its a hard book to recommend because of the intense emotional impact that it has, yet in that respect, it should be read. Some of the acts described in the book will make you wince or turn your stomach, others hit some primal fears that exist in all of us and make the book impossible to forget.
The Girl Next Door is a true horror book and represents, in my opinion, one of the best works in that genre.
Read the book if you feel you have the stomach for it and, for once, do believe the hype.
As a footnote, I would also recommend The Basement by Kate Millett, a semi fictional account of the true story, told from the imagined perspectives of some of the people concerned. It adds further impact to the events described in The Girl Next Door.